1. Field of the Invention
This invention relates to the field of disc media, and more particularly to a type of disc which is constructed by assembling or joining two disc halves together to form a composite disc.
2. Brief Description of the Prior Art
Current commercial videodiscs are constructed from two disc halves joined together by an adhesive resulting in what may be termed a composite disc. There are many advantages of using two disc halves as opposed to molding a single disc. The most important advantage is that the resulting laminated disc resists warping based on its symmetry. Most recorded or recordable discs have some reflective or ablative surface on one side of the disc, and the other side of the disc serves as a window for the passage of a recording and/or reading laser beam. The metal reflecting layer in such discs acts as a moisture barrier such that the distribution of moisture throughout the disc is uneven, and this again is contributory to a possible severe warping problem. A composite construction avoids these physical problems and also allows, at least in the consumer disc field, single-sided disc halves to be recorded independently and good disc halves to be joined together with an adhesive to form a two-sided disc with higher overall yields.
To join the two disc halves together, in the past an amount of contact cement was sprayed onto one or both of the discs to be joined, and the adhesive was allowed to cure, as necessary. The two disc halves were then brought together and pressed to form the completed disc. An alternative way of joining disc halves is to provide a thin sheet of adhesive stretched out over one of the disc halves, and apply pressure to compress the two disc halves together with the sheet of adhesive therebetween.
In the normal assembly of the disc halves of recordable disc media, a sheet of adhesive is rolled onto a dummy disc, and then the dummy and the recordable disc are rolled together. In the process, air bubbles and contaminants which may have attached themselves to the adhesive sheet prior to application to the discs, may be sandwiched between the discs. These inclusions will produce bumps in the recordable surface which, if they are large enough, will no longer fall within the focus range of the lens, leaving the area unrecorded.
The basic problem with composite discs of the prior art is that, because the discs are bumpy, the focus servo cannot maintain focus on the information layer.
Although this problem has been known for several years, there has not been a great deal of interest in improving the flatness characteristics of the discs, since consumer players are able to cope with such problems quite readily. In a consumer player, it is only necessary to focus a lens of low numerical aperture (NA) and corresponding large depth of focus in order to recover the information. Because the numerical aperture of the objective lens in consumer players is lower than that of industrial recorder/players, the consumer playback-only system is less sensitive to out-of-flat surface defects.
Industrial recorder/player instruments use objective lens assemblies of high numerical aperture to produce small recorded marks at relatively low recording power. Unfortunately, high NA lenses are heavy and have a shallow depth of focus, so that maintaining focus on a bumpy disc is very difficult and requires a very high performance focus servo.
The unflatness of discs is primarily due to the trapping of air bubbles and contaminants between the disc halves as the two disc surfaces are brought together. Obviously the method of assembling disc halves should be done in a clean environment. However, even in the cleanest of environments (reducing the trapping of contaminants), trapped bubbles can cause information surface irregularities on the order of a few microns which is disastrous when considering that the track width on a recordable videodisc, for example, is on the order of 0.6 microns. These small surface bumps require the objective lens to undergo high vertical acceleration in order to maintain focus, and, as previously indicated, industrial recorders simply cannot accommodate these kinds of defects. As a result, the yield of usable discs is low, contributing to their higher costs.
It is thus clear that a composite disc and method of making the same in which the aforementioned surface defects can be eliminated would be highly desirable. The present invention provides such an improved disc.